THE ENGINES: WHAT WENT WRONG?
stu davies comments
The motorboat in question is fitted with twin Perkins 4108 engines mated to stern drives. These engines were built in the 1960s. The Perkins 4108 is a good engine, solid, dependable and thousands of them were built and used in boats, tractors, fork lifts, generators – and in fact anything that needed a dependable workhorse engine with spares readily available worldwide.
The 4108 was marinised by a few manufacturers by fitting seawatercooled heat exchangers/intercoolers on the port side of the engine where the exhaust comes out of the cylinder head.
The injector pump is fitted midway down the engine on the port side, underneath the heat exchanger.
Like all marinised engines, the heat exchanger cooling seawater is injected into the exhaust elbow aft of the heat exchanger from where it goes along the exhaust pipe to cool it and that is where issues can arise.
Over the years carbon and salt can build up in the exhaust elbow where the waste cooling seawater is injected, leading to blockages. Also the hot seawater combined with the hot exhaust gases provides the perfect environment for corrosion to take place in the elbow.
I suspect from Roger’s story that the elbow/s were on their last legs – severely corroded and just waiting for
the most perfect, inopportune moment to let go. Roger refers to the overheating on the port engine and of course that would then lead to low oil pressure readings as well. This could have been caused by a loose or broken connection, a bad water pump impeller or something similar.
The fact it stopped and wouldn’t restart probably indicates that the engine seized from overheating.
The reference to flames coming out of the ‘loose manifold’ and the subsequent survey indicates that the exhaust elbows perforated, and exhaust flames would have been coming out of there and the waste raw cooling water would also have been coming in to the boat from the same place.
The reference to the ‘glowing red’ injector pump I think can be put down to the stressful situation they were in. As explained previously the injector pump is fitted on the port side of the engine under the heat exchanger. Even on a perfect engine the heat exchangers get seriously hot! Trying to reach past it and underneath it to reach the stop lever would not be something I’d like to do!
Normally the exhaust gases go out of the engine down the steel elbow and then in to a silencer, (some of which are plastic) and then rubber hose to exit the boat.
Could this start a fire?
The raw water injection is at the elbow and the water both cools the plastic silencer and rubber hose and muffles the exhaust sound. If the elbow corrodes through, you get the situation Roger describes with the raw water now going into the boat, and flames coming out of the manifold. Can this cause a fire?
With no cooling water and the plastic silencer and rubber exhaust hose subject to the exhaust gases, which as he said are seen as flames, the silencers melt and the rubber hose chars. At this point there is a real possibility that fire can take hold.
Even if it doesn’t come from there, depending on the proximity of flammable objects, it could start an engine room furniture fire.
This sad story is a perfect example of why, at the very least, you need a knowledgeable person to cast an eye over things first and then consider having a professional survey done.
‘Over the years carbon and salt can build up in the exhaust elbow where the waste cooling seawater is injected, leading to blockages’